M01.05.006 Sternum

The sternum, or breastbone, is a flat bone located in the midline of the anterior thorax. Shaped like a “T,” it serves as a vital component of the bony thoracic wall, protecting internal thoracic organs such as the heart, lungs, and esophagus.

Structure and Parts of the Sternum

The sternum consists of three distinct parts:

1. Manubrium
2. Body
3. Xiphoid Process

Each part has unique features and functions that contribute to the sternum’s role in the thoracic skeleton.

1. Manubrium

The manubrium is the most superior section of the sternum, trapezoid in shape, and features several notable landmarks:

• Jugular Notch: A concave depression visible beneath the skin at the top of the manubrium.
• Sternoclavicular Joints: Articulate with the medial ends of the clavicles at cartilage-lined fossae.
• Rib Articulations:
  • Facet for the first rib.
  • Demifacet for part of the second rib.
• Sternal Angle (Angle of Louis):
  • Formed by the articulation with the body of the sternum.
  • Palpable transverse ridge used as a reference to identify the 2nd rib.

2. Body

The body is the largest and most elongated part of the sternum, articulating with the manubrium superiorly and the xiphoid process inferiorly.

Features:

• Articular Facets: For ribs 3-6.
• Demifacets: For parts of the 2nd and 7th ribs.
• Flat surface ideal for the attachment of muscles and ligaments.

3. Xiphoid Process

The xiphoid process is the smallest and most inferior portion of the sternum.

Key Characteristics:

• Shape and Size: Variable among individuals.
• Ossification: Initially cartilaginous, it ossifies completely around age 40.
• Rib Articulation: Sometimes articulates with the costal cartilage of the 7th rib.
• Location: Tip aligns with the T10 vertebra.

Clinical Correlations

Fractures of the Sternum

Sternal fractures are rare but associated with severe chest trauma, such as in vehicle accidents.

Key Points:

• Most Common Site: Manubriosternal joint.
• Type of Fracture: Typically comminuted (broken into multiple pieces).
• Muscle Protection: Fracture fragments often remain undisplaced due to pectoralis muscle attachments.
• Mortality Risk: High (25-45%), not from the fracture itself but from associated heart and lung injuries.

Investigations:

• X-ray: Common initial imaging.
• CT and Ultrasound: Useful for assessing visceral injuries.

Table: Parts of the Sternum and Their Features

Points to Remember

1. The sternum is a flat bone forming the anterior midline of the thoracic cage.
2. It comprises three parts: manubrium, body, and xiphoid process.
3. The sternal angle is a key anatomical landmark for rib identification.
4. Sternal fractures often result from high-impact trauma and are associated with visceral injuries.
5. Complete ossification of the xiphoid process occurs late in life, around 40 years of age.

Bibliography

1. Moore, K. L., Dalley, A. F., & Agur, A. M. R. Clinically Oriented Anatomy. Wolters Kluwer.
2. Gray, H. Gray’s Anatomy: The Anatomical Basis of Clinical Practice. Elsevier.
3. Standring, S. Gray’s Anatomy: The Anatomical Basis of Clinical Practice. 41st Edition.